- sys-devel/binutils-2.25.1-r1::gentoo (masked by: package.mask)
/usr/portage/profiles/package.mask:
# MichaÅ GÃ³rny <mgorny@gentoo.org>, Andreas K. HÃ¼ttel <dilfridge@gentoo.org>,
# Matthias Maier <tamiko@gentoo.org> (21 May 2017)
# These old versions of toolchain packages (binutils, gcc, glibc) are no
# longer officially supported and are not suitable for general use. Using
# these packages can result in build failures (and possible breakage) for
# many packages, and may leave your system vulnerable to known security
# exploits.
# If you still use one of these old toolchain packages, please upgrade (and
# switch the compiler / the binutils) ASAP. If you need them for a specific
# (isolated) use case, feel free to unmask them on your system.

- sys-libs/glibc-2.21-r2::gentoo (masked by: package.mask)
For more information, see the MASKED PACKAGES section in the emerge
man page or refer to the Gentoo Handbook.

Remove all packages that are no longer in tree. As a general rule, Gentoo maintainers do not create/retain blockers for packages that no longer exist, so Portage will offer you substantially less help in resolving conflicts that involve dead packages.

It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously. If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.

It may be possible to solve this problem by using package.mask to
prevent one of those packages from being selected. However, it is also
possible that conflicting dependencies exist such that they are
impossible to satisfy simultaneously. If such a conflict exists in
the dependencies of two different packages, then those packages can
not be installed simultaneously.

For more information, see MASKED PACKAGES section in the emerge man
page or refer to the Gentoo Handbook.

1. It looks I am getting gcc-6.4. Does this mean I will get C++11 for free and will not need to supply this option thru the "-std=c++11" flag?
2. Do you think it will be better to run preserve-rebuild after world merges or wait until KDE5 will be installed? My take is to wait, but whats your insight?

The following USE changes are necessary to proceed:
(see "package.use" in the portage(5) man page for more details)
# required by sys-devel/clang-3.9.1-r100::gentoo
# required by dev-util/kdevelop-5.1.1::gentoo
# required by kdevelop (argument)
>=sys-devel/llvm-3.9.1-r1 clang

Now for some reason I will now need llvm installed, which I don't need. So I tried: